But over time that dense atmosphere leaked away, says Richard Zurek, co-investigator on the MAVEN mission and a scientist with NASA's Jet Propulsion Laboratory in Pasadena.

Zurek says solar wind may be partially to blame. Imagine a cloud of fast moving charged particles spit out by the sun.

"Those particles have energy and as they collide with the atmospheric molecules around Mars: They give some of that energy to the molecules," Zurek explained.

The theory is that once particles in the Martian atmosphere are charged up by this solar wind, they may have enough energy to escape the pull of Mars' gravity and float off into space.

Zurek says Earth is also regularly bathed in solar wind, but — unlike Mars — our planet has a strong magnetic field protecting the atmosphere. Mars had a similarly strong magnetic field in the past, Zurek, but somehow it diminished.